1. Field of the Invention
The present invention relates to electronic components such as multilayer ceramic (MLC) electronic components and, more particularly, to the use of high temperature superconductors as wiring in the multilayer ceramic circuit structure.
2. Description of Related Art
The scale of integration in integrated circuits has steadily increased over the years resulting in integrated circuits with extreme complexity and capable of very comprehensive functions. While any electronic component may be used with the subject invention, the description will be directed to a structure which uses a plurality of layers of glass, ceramic or other insulating material of relatively high thermal conductivity with conductors formed on the respective surfaces and in through-holes (vias) in the layers. These components are typically termed multilayer ceramics (MLC) and since circuits constructed in this way are three dimensional, a high degree of complexity is possible. Even with this increased complexity and circuit density however, there is still a need for large size and/or densely wired high end modules for use in low temperature computing. High temperature superconductor wiring has been proposed for use in the fabrication of the electronic components.
In conventional MLC structures, the conductors are usually formed by applying the desired pattern on the substrate using a conductive paste. The conductive paste pattern is usually formed by extruding the paste, which is usually highly viscous, through a stencil or mask such as a perforated sheet by passing an extrusion nozzle over the mask located on the ceramic layer or greensheet of the MLC. This has not been found acceptable for high temperature superconductors such as La--Sr--Cu--O, Y--Ba--Cu--O and Bi--Sr--Ca--Cu--O materials since conventional fabrication techniques degrade the superconductor material resulting in a reduction in critical current density and lowering of the superconductor transition temperature making operation of the component at liquid nitrogen temperatures virtually impossible.
In general, a multilayer ceramic semiconductor package (MLC) is formed by stacking and bonding together flexible paper-like sheets commonly referred to as ceramic greensheets. Greensheet segments of desired size and configuration are punched to provide via holes and, by a screen printing technique, a conductive paste fills the via holes and a conductive circuit pattern is applied to the face of the greensheet as required. Such patterned greensheets, after screening, are assembled in a stack, pressed and subsequently sintered in an oven at a relatively high temperature. Upon sintering, the organics such as binders, dispersants, plasticizers, thixotropes, solvents, etc. are burned off providing a rigid unitary ceramic body having interior interconnected conductive patterns.
There are many types of MLC conductive pastes which are screened onto the greensheet circuit to form the patterns. In general, a metallic paste such as a molybdenum paste comprises molybdenum power, a solvent and a binder. A rheological modifier and/or dispersing agent may also be employed to control viscosity and pattern formation. The concentration of the various ingredients may vary widely with the proviso that a paste is formed which has a proper viscosity and which may be effectively applied to the greensheet or other surfaces by screening or other patterning techniques.
Typical solvents for the metal paste include n-butyl carbitol acetate, .sigma. terpineol, n-butyl carbinol and the like. The binder includes materials such as ethyl cellulose, polyvinylbutyral and the like. The metal material has a fine particle size generally less than 10 microns on average. Typically, the metal pastes are prepared by adding the ingredients to a mill and mixing the ingredients until a homogenous paste is obtained.
In general, a complete conventional patterning process for greensheets may be described as follows. A ceramic slurry is prepared by mixing alumina (ceramic) powder, organic binder, plasticizer and solvent at a given mixing ratio in a conventional manner. The slurry is shaped into a tape by means of a tape caster and thereafter the resulting tape is dried and cut into a given length to form a plurality of unsintered soft ceramic sheets or so-called "greensheets".
Via holes are then punched in the sheets and circuit patterns are printed on the surface of each greensheet and the vias filled with a metallizing ink or paste using a screen (mask) printing process. The metallizing paste, as noted above, is typically a molybdenum paste although other metallic pastes such as tungsten may also be used. The sheets are then dried, stacked, laminated and thereafter sintered to form the MLC substrate.
The fabrication of multilayer ceramic substrates is described in U.S. Pat. Nos. 4,137,628; 4,803,110 and 4,902,371 and in Microelectronics Packaging Handbook, Edited by R. R. Tummala and E. J. Rymaszewski, 1989, pages 476-493. The disclosures of the above patents and references are incorporated herein by reference.
Bearing in mind the problems and deficiencies of the prior art with regard to the use of high temperature superconductors as wiring in MLC's, it is therefore an object of the present invention to provide a method for the fabrication of electronic components including MLC electronic components using high temperature superconductors as wiring and other circuitry and to fill the vias.
An additional object of the present invention is to provide an electronic component such as an MLC electronic component made using a high temperature superconductor material as a circuit forming material in the component.
Other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.